Fluids (i.e., gases and liquids) are used in many manufacturing industries. Within a manufacturing process, fluids are typically dispensed with precision of flow rate, timing or both. Generally, fluid dispensing equipment has the ability to adjust the flow rate by means of flow control valves. A flow control valve regulates the opening of a flow path (i.e., an orifice) to suit the necessary flow rate.
In addition to the orifice, the flow rate also depends on the properties of the fluid, as well as its temperature and pressure. For a given fluid at steady temperature and pressure, stable flow rates may be determined simply by the setting of the flow control valve. However, in many practical flow control systems, the combination of temperature and pressure instability, as well as the finite precision and accuracy of the flow control valve, may prohibit the attainment of adequately stable and/or repeatable flow rate. In addition, downstream from the flow control valve, a plurality of valves, sensors and other elements may also impact flow rate stability. In such instances, a flow control valve may incorporate a flow sensor and a tunable flow control valve to pursue the desired flow rate. For that purpose, the flow control system drives the tunable flow control valve to adjust the actual flow rates, to be equal to the desired flow rate. This self-correcting fluid delivery method has been successfully adapted for automatic manufacturing in many different industries.
Conventional flow control valves implement a variety of variable-orifice designs. Commonly, the orifice is confined between two surfaces. One of the surfaces comprises a fluid flow inlet, commonly known as a “valve seat.” A second surface is mechanically actuated to vary the gap between these surfaces and adjust the orifice. The second surface is often attached to a precision controlled mechanical actuator such as a precision screw, a proportionally controlled solenoid or a proportionally controlled piezoelectric actuator. Typically, the impact of the gap on the flow rate is strongly nonlinear, especially for small gaps. Given the nonlinearity, as well as limitations of mechanical stability and precision, most manually driven, precision screw-based valves resort to designs that increase the surface area of the gap. This increased area provides sufficient flow resistance at somewhat larger gaps to reduce the impact of nonlinearity and the effects of mechanical imperfections and screw backlash, thereby improving flow control.
One popular design, for example, implements a motion controlled tapered needle and a matching cavity. This “needle valve” design enables reasonably stable and repeatable manually controlled valves that are capable of reliable flow control down to about 2% of the maximum flow, wherein maximum flow means the flow of a fully open valve. However, highest performing “metering needle valves” are prone to significant wear of the needle over the needle cavity. This wear is especially pronounced when controlling the small gaps that provide flow rates lower than 5% of the maximum flow. Such wear results in both reduced valve performance and the generation of particles. Particles that are generated in the flow control system and consequentially delivered into the process via streams of fluids are detrimental to the quality of many industrial manufacturing applications and are therefore undesirable. Accordingly, needle valves are mainly implemented for applications that do not require frequent flow adjustment and are not very sensitive to particulates. Moreover, they are best used when not pushed to control flow rates below about 5% of their total range.
The precision of a manual flow control valve is adversely impacted by temperature and pressure instabilities as well as potentially fluctuating impact of downstream components. Therefore, they have growingly become inadequate for precision manufacturing. Instead, flow control valves (also known as Mass Flow control valves—MFCs) can actively and controllably adjust the orifice gap to yield desired flow rates regardless of mechanical precision and stability limitations. Likewise, they can correct for temperature and pressure fluctuations or the impact of downstream components. They are also fully compatible with factory automation and quality control systems, hence, their growing widespread popularity. Within MFCs, flow control valves with gap controlling actuators adjust the orifice between a fixed plane and the moving end of the actuator, wherein a fluid entry port, commonly called a “valve seat,” is defined on the fixed plane. Hence, the actuator is immersed in the fluid with potential fluid contamination, actuator corrosion, jamming and particle generation. To address this issue, some designs provide a metallic diaphragm disposed between the actuator and the valve seat, in compatibility with Ultra High Purity (UHP) standards. For example U.S. Pat. No. 8,162,286 to Sawada et al. and entitled “Piezoelectric Driven Control Valve” discloses a flow control valve with a piezoelectric actuator and a UHP design. Likewise, U.S. Pat. No. 5,447,173 to Kazama et al. and entitled “Mass Flow Controller, Operating Method and Electromagnetic Valve” discloses a flow control valve with a solenoid actuator and UHP design.
FIG. 1 depicts a UHP-compliant flow control valve 100. A dome-shaped metallic diaphragm 101 creates an all-metallic valve chamber 102 over valve seat 103. Valve seat 103 is located substantially across from the concave center of diaphragm 101. The orifice 104 is defined between valve seat 103 and diaphragm 101. Within valve chamber 102, a fluid outlet port 105 is also formed. Diaphragm 101 seals the fluid within valve chamber 102 to prevent fluid-actuator contact. The gap between diaphragm 101 and valve seat 103 is reduced when actuator 106 deforms diaphragm 101 towards valve seat 103. Similarly, the gap is increased when a retreat of actuator 106 allows diaphragm 101 to spring back away from valve seat 103. Also indicated in FIG. 1 are flow sensor 107, controller 108, enclosure 109, inlet fitting 110 and outlet fitting 111.
Typically, conventional actuators are electrically driven in one direction and mechanically returned in the other direction by a spring in the case of solenoids, or by strain discharge in the case of piezoelectric actuators. Generally, well designed actuators can provide nearly linear position change per electrical drive (current for solenoids and voltage for piezoelectric actuators). As a result, linearly responding actuators may tune the orifice gap linearly. Nevertheless, linear gap tuning produces nonlinear flow change. Ideally, actuators should be able to correct a difference between actual flow rate and the desired (set point) flow rate instantaneously and accurately. Likewise, they should also be able to respond to a set point change with speed and precision.
Automated flow control valves typically apply closed-loop, proportional-integral (PI) or proportional-integral-derivative (PID) algorithms to alter the gap and adjust the flow to match the set point. However, both PI or PID control algorithms, as well as other control algorithms known in the art, are not suitable for accurate and responsive control of nonlinear systems. Particularly, nonlinearity makes flow control parameters flow dependent. That means that, for a given set of flow control parameters (i.e., particular PI or PID constants), controllers cannot accurately control the actual flow to match a set point beyond the single flow rate that was selected to initially tune the system and extract the PI or PID constants. Using these same flow control parameters to control other flow rates often yields erroneous and sometimes oscillatory flow rates, as well as substantially slower response. Likewise, erroneous and oscillatory flow rates may also be driven by temperature and inlet pressure change, drift, or other fluctuations. Similarly, many applications require the mixing of several flow-controlled sources of different fluids. Some systems also manipulate the mixing ratio during the process. Lastly, many mixing manifolds increase the pressure downstream from flow control valves. This increased downstream pressure acts to reduce the flow per given orifice, causing flow control valves to react to increase the orifice and match the actual flow to the set point. However, that reaction repositions the entire flow dependent PI or PID parameters. Due to nonlinearity, the shifts in the flow to gap dependence detunes the control system into lower performance, which results in sluggish response, increased error and tendency for oscillations. To overcome this problem, the flow control valves should ideally be tuned at the mixing condition. This tuning has to be repeated iteratively to converge all flow control valves into accurate and non-oscillatory control. If the process requires mixing ratio changes, new control constants are preferably deduced for all flow control valves and applied as part of the change. Even so, however, the mixing ratio is nevertheless undefined and unstable during transitions between different settings.
These fundamental deficiencies may be partially addressed by invoking actuator and orifice designs with reduced nonlinearity and controllers that apply microprocessors for flow versus actuator motion corrections (“gain scheduling”) as part of the closed-loop control. Likewise, implementing temperature compensation and integrating inlet pressure control may tame the adverse impact of fluctuating or drifting temperature and/or inlet pressure. In some cases, when controlling fluids with relatively high inlet pressures, a highly restrictive outlet orifice may effectively suppress the destabilizing impact of downstream components and/or other flows in the system. It is also recognized that flow control valves perform best when the size of the orifice is optimized to the application given the flow range, the type of fluid, the temperature and the entire process system. Given these improvements, sophisticated modern flow control valves may operate adequately over a range of flow rates around 10-90% of maximum flow for a wide range of applications. At the same time, these modern flow control valves can mitigate the slow and oscillatory response at both the low and the high 20-30% ends of the range, as well as erroneous transient performance at flow rates that are outside a very narrow range of the most optimized flow rate. In addition, in order to cover a wider range, systems may use multiple different flow control valves with different ranges. That said, range limitations still create the need for many different and distinctive models of flow control valves, and thereby substantially increase inventory size and cost for manufacturers.
Flow control valves are also applied to control the pressure within delivery manifolds wherein the flow sensor is substituted for a pressure sensor and the flow control device is tasked with tuning the actual pressure to a set point pressure. These pressure controllers find usage in a variety of applications, such as fluid delivery into Atomic Layer Deposition (ALD) systems. In these particular applications, the pressure controllers allow pressure-controlled gas to be made available at the inlets of fluid delivery valves that drive the ALD processing. Such fluid delivery valves for ALD are discussed in, for example, U.S. Pat. No. 7,744,060 to Sneh and entitled “Fail-safe Pneumatically Actuated Valve with Fast Time Response and Adjustable Conductance,” which is hereby incorporated by reference herein. When a fluid delivery valve is opened, precisely metered delivery is established with a flow rate that is determined by the controlled inlet pressure and the conductance of the ALD valve. That delivery is shaped as a pulse. Accordingly, the flow control valve of the pressure controller must accurately respond to the pulse by quick flow rate changes, essentially going between a zero flow rate when pressure is at the pressure set point and a flow rate sufficiently high to restore the pressure back to the set point during and just after a pulse. The precision of this pressure control is vital for efficient usage of reactive gas, as well as to the implementation of Synchronously Modulated Flow and Draw (SMFD) ALD, which is discussed in U.S. Pat. No. 6,911,092 to Sneh and entitled “ALD Apparatus and Method,” which is also hereby incorporated by reference herein. SMFD ALD implements fast ALD processes with <500 millisecond (ms) cycle times and recovery times between successive pulses trending to below 150 ms. Conventional pressure controllers struggle with such demanding applications.
One other challenging application implements flow control valves as pressure controllers to control the pressure of inert gas that is used to provide improved thermal contact to process heaters and improve their isolation from process effluents. In particular, fragile heaters may be disposed inside a heating chuck wherein heater to chuck contact and/or complete sealing of the heaters inside the chuck is not possible. Accordingly, helium gas (He) is applied to assist in heat transfer and provide positive flow out of the chuck space to negate the penetration of harsh process chemicals. Ideally, helium pressure inside the chuck should be kept at the same value during idle, process and transitions between idle and process so as to promote stable chuck temperature. A typical sequence of idle, part-handling, idle, transition, process, transition challenges the pressure controller to adapt quickly to a sequence of flow-1, no-flow, flow-1, transition to flow-2, flow-2, transition to flow-1, wherein flow-2 is smaller than flow-1 given the impact of the process pressure. During part transfer, a shutoff valve is typically turned off, dropping the flow of helium to zero. It is the objective of the flow control valves to quickly react to these varying conditions so as to maintain the pressure at set value during the entire cycle while enduring the impact of significant flow rate changes. Conventional flow control valves struggle with this application.
In a similar application, inert gas is directed to prevent process fluid from reaching the backside of a wafer. In this application, inert gas is applied into the gap between a chuck and the backside of the wafer. A first pressure sensor is applied to obtain the pressure in the process chamber. A second pressure sensor is applied to obtain the pressure at the inert gas delivery line. In this case, the flow control valve is tasked with controlling the pressure differential between the second and the first pressure sensors to a given pressure differential set point, and to ensure that, independent of process pressure variability by design or due to imperfection, there is always a set point pressure differential to negate the flow of process chemicals into the gap. Here too, conventional flow control valves struggle with such a demanding application.
In some other common applications, precision controlled motion of pneumatic or hydraulic actuators is used to propel parts handling, robotic motion, stamping, etc. In these applications, flow-controlled fluids (e.g., air or hydraulic fluids) determine the speed of these motions. Often the speed of robotic arms and parts handling has to follow intricate and complex profiles with well-defined acceleration and deceleration profiles. In addition, some of these motions have to be able to perform their tasks under different or time-variable load conditions. To accommodate these requirements, flow control valves with adequate precision and fast response across wide ranges of flow rates and with the ability to accommodate a wide range of loads are needed. Existing systems struggle with many of these applications, in particular, when speed is of the essence.
Conventional flow control valves may also struggle to provide reliable operation at high temperatures, to provide reliable low and zero flow performance, and to provide mechanisms for fail safety, that is, mechanisms to automatically shut off when unexpected system conditions are encountered. With few exceptions, conventional flow control valves do not provide UHP construction with proper resistance to fluid contamination, valve corrosion, particle generation and jamming. These flow control valves with actuators immersed in the fluids are not suitable to control the flow of most liquids. Most flow control valves comprise a very small orifice as part of the means to be able to control relatively low flow rates from relatively high inlet pressures. These permanent flow restrictors adversely slowed down the rate of purging and decontaminating flow and pressure controllers and their manifolds prior to component replacement and/or the performance of maintenance.
In recent decades, many manufacturing processes have strived to improve efficiency, increase quality and reduce cost and waste. This trend growingly emphasizes reliable and precise automation as well as the ability to use manufacturing equipment as much as possible over a wide range of different processes. Within that trend, flow and pressure controllers with improved speed and precision over a wide range of flow rate, inlet pressure and ambient conditions, are essential for optimal, low waste and repeatable processing.
For the foregoing reasons, there is a need for new designs for flow control valves that address the above-identified deficiencies.